Method for fabricating semiconductor device with different gate oxide compositions

ABSTRACT

First, an isolation region is formed on a surface portion of a semiconductor substrate of silicon, thereby defining first and second regions, which are isolated from each other by the isolation region, on the semiconductor substrate. Next, a tantalum oxide film is formed in the first region on the semiconductor substrate. Then, a silicon dioxide film is formed in the second region on the semiconductor substrate by heat-treating the semiconductor substrate within an ambient containing oxygen as a main component. Subsequently, first and second gate electrodes are formed on the tantalum oxide and silicon dioxide films, respectively. Thereafter, first and second gate insulating films are formed by etching the tantalum oxide and silicon dioxide films using the first and second gate electrodes as respective masks.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device and a method forfabricating the same.

In recent years, semiconductor devices have been drastically downsizedand increasingly required to perform an even wider variety of functionsconcurrently. Considering such a state of the art, it has become moreand more necessary to change the thickness of a gate insulating film,which is provided for multiple MOS transistors on the same semiconductorsubstrate, from place to place. Specifically, to make all of thesetransistors operate reliably, the gate insulating film should bepartially thickened for some transistors operating at relatively highvoltages, but be partially thinned for other transistors operating atrelatively low voltages. Another situation where the thickness of thegate insulating film has to be changed arises when several kinds ofthreshold voltages need to be prepared for these transistors.

In addition, as semiconductor devices have been miniaturized, thenecessity of reducing the thickness of a gate insulating film has alsobeen increasing day after day. For that purpose, according to a proposedtechnique, a thinner gate insulating film was provided by making theinsulating film of tantalum oxide, which has a relative dielectricconstant higher than that of a silicon dioxide film used conventionally.Specifically, the relative dielectric constant of the tantalum oxidefilm is about 25, whereas that of the conventional silicon dioxide filmis about 3.9. Accordingly, the dielectric strength of the tantalum oxidefilm is about 6 times as high as that of the silicon dioxide film.

If a voltage as high as 1.5 V or more is applied to the gate electrode,however, then the tantalum oxide film is hard to use as the gateinsulating film anymore in view of the leakage current characteristicthereof. Thus, in such a situation, two types of transistors, i.e.,transistors including a tantalum oxide film as gate insulating film andtransistors including a silicon dioxide film as gate insulating film,have to be formed on the same semiconductor substrate.

Hereinafter, a method for fabricating a semiconductor device, whichincludes a transistor with a silicon dioxide gate insulating film and atransistor with a tantalum oxide gate insulating film on the samesemiconductor substrate, will be described as first prior art examplewith reference to FIGS. 11(a) through 11(d). In FIGS. 11(a) through11(d), the tantalum oxide gate insulating film will be formed in a firstregion on the left-hand side, and the silicon dioxide gate insulatingfilm will be formed in a second region on the right-hand side.

First, as shown in FIG. 11(a), isolation regions 11 are defined on thesurface of a semiconductor substrate 10 of silicon, and then a tantalumoxide film 12 is deposited to a thickness of 10 nm, for example, in thefirst and second regions of the semiconductor substrate 10.

Next, as shown in FIG. 11(b), a mask 13 is made out of a silicon dioxideor resist film over the first region of the semiconductor substrate 10,and the tantalum oxide film 12 in the first region is etched away usingthe mask 13.

Then, as shown in FIG. 11(c), a silicon dioxide film 14 is formed to athickness of 5 nm, for example, in the second region of thesemiconductor substrate 10.

Subsequently, a conductor film is deposited over the tantalum oxide andsilicon dioxide films 12 and 14 and then patterned into gate electrodeshapes. In this manner, first and second gate electrodes 15 and 16 areformed on the tantalum oxide and silicon dioxide films 12 and 14,respectively, as shown in FIG. 11(d). Thereafter, the tantalum oxide andsilicon dioxide films 12 and 14 are etched using the first and secondgate electrodes 15 and 16 as respective masks, thereby forming first andsecond gate insulating films 17 and 18.

If the tantalum oxide film 12 is used as the gate insulating film,however, then a silicon dioxide film with a relative dielectric constantlower than that of the tantalum oxide film 12 is formed in the interfacebetween the tantalum oxide film 12 and the semiconductor substrate 10 ofsilicon during a subsequent heat treatment process. As a result, thetotal relative dielectric constant of the gate insulating film adverselydecreases.

An alternative method for fabricating a semiconductor device accordingto a second prior art example, which was proposed to avoid this problem,will be described with reference to FIG. 12. As shown in FIG. 12, beforethe tantalum oxide film 12 is deposited, the surface of the underlyingsemiconductor substrate 10 is nitrided at 900° C. for about 60 secondswithin a rapid thermal annealing furnace, thereby forming anitrogen-containing silicon layer 19 on the surface of the semiconductorsubstrate 10.

Also, just after the tantalum oxide film 12 has been deposited, thetantalum oxide film 12 is amorphous and contains a lot of carbon. Forthese and other reasons, the tantalum oxide film 12 as deposited shouldbe subjected to oxidation or crystallization heat treatment.Furthermore, when the tantalum oxide film 12 is used as gate insulatingfilm, the gate electrode is made of a metal such as Ti, W or TiSi_(x).Thus, it is effective to deposit a TiN, WN or TaN film as barrier metallayer in the interface between the gate insulating film and the gateelectrode.

The material of the silicon dioxide film has also been modified recentlyto cope with the demand for further reduction in thickness of the gateinsulating film. Specifically, a silicon oxynitride film, which isobtained by introducing nitrogen into a silicon dioxide film, has beenused more and more often instead of the silicon dioxide film essentiallyconsisting of silicon and oxygen only. Examples of known methods forforming the silicon oxynitride film include: (1) forming a silicondioxide film and then heat-treating it within an ammonium or nitrogenoxide ambient; (2) forming a silicon nitride film and then heat-treatingit within an oxygen ambient; (3) conducting a heat treatment within anambient in which nitrogen or ammonium and oxygen are mixed; and (4)directly heat-treating the surface of a silicon substrate, on which nofilm has been deposited yet, within a nitrogen oxide ambient.

In a storage capacitor portion of a semiconductor storage device such asDRAM, an ONO film, which is a stack of silicon dioxide and nitridefilms, has been used.

However, to cope with the downsizing of capacitors, a tantalum oxidefilm has recently attracted a lot of attention as a new material for acapacitive insulating film. If the tantalum oxide film is used as aninsulating film for a storage capacitor portion, the tantalum oxide filmhas to be subjected to a nitriding or oxidation/crystallization processbefore or after the deposition as in the gate insulating film.

Also, a semiconductor storage device has been more and more requiredlately to process several information items at a time by using aplurality of regions with different quantities of charge.

If an MOS transistor with the silicon dioxide gate insulating film andanother MOS transistor with the tantalum oxide gate insulating film areformed on the same semiconductor substrate in the above-describedmanner, then the number of masking process steps increases. In addition,since the heat treatment should be conducted separately to form thetantalum oxide and silicon dioxide films on the semiconductor substrate,the dopant introduced into the semiconductor substrate might diffuseexcessively to affect the basic electrical characteristics of thetransistors. As a result, it is difficult to ensure desired performancebecause MOS transistors of a very small size cannot be formed in such acase.

This problem happens. frequently when multiple MOS transistors withdifferent types of gate insulating films are formed on the samesemiconductor substrate. e.g., when an MOS transistor with a gateinsulating film of silicon dioxide or tantalum oxide and an MOStransistor with a gate insulating film of silicon oxynitride are formedon the same semiconductor substrate.

A similar statement is also applicable to a semiconductor storagedevice. Specifically, if two types of capacitive insulating films,namely, tantalum oxide and silicon dioxide films, are prepared to storemutually different quantities of charge in a storage capacitor portion,then the number of masking process steps and the total number offabricating process steps also increase. In addition, since the heattreatment should be conducted an increased number of times, the overallelectrical characteristics of the device disadvantageously deteriorate.

SUMMARY OF THE INVENTION

In view of the foregoing respects, a first object of the presentinvention is forming MOS transistors with different types of gateinsulating films without increasing the numbers of heat-treating andmasking process steps. A second object of the present invention isforming a capacitor with different kinds of capacitive insulating filmswithout increasing the numbers of heat-treating and masking processsteps.

To achieve the first object, a first method for fabricating asemiconductor device according to the present invention includes thesteps of: forming an isolation region on a surface portion of asemiconductor substrate of silicon, thereby defining first and secondregions, which are isolated from each other by the isolation region, onthe semiconductor substrate; forming a tantalum oxide film in the firstregion on the semiconductor substrate; forming a silicon dioxide film inthe second region on the semiconductor substrate by heat-treating thesemiconductor substrate within an. ambient containing oxygen as a maincomponent; forming first and second gate electrodes on the tantalumoxide and silicon dioxide films, respectively; and forming first andsecond gate insulating films by etching the tantalum oxide and silicondioxide films using the first and second gate electrodes as respectivemasks.

According to the first method for fabricating a semiconductor device, asemiconductor substrate, on which a tantalum oxide film has been formedin a first region, is heat-treated within an ambient containing oxygenas a main component. Thus, the tantalum oxide film is left in the firstregion, while a silicon dioxide film is newly formed in a second region.Thereafter, first and second gate insulating films are formed by etchingthe tantalum oxide and silicon dioxide films using first and second gateelectrodes as respective masks. In this manner, the first and secondgate insulating films can be formed out of the tantalum oxide andsilicon dioxide films, respectively, on the same semiconductor substratewithout increasing the number of heat-treating or masking process steps.

To achieve the first object, a second method for fabricating asemiconductor device according to the present invention includes thesteps of: forming a plurality of isolation regions on respective surfaceportions of a semiconductor substrate of silicon, thereby definingfirst, second and third regions, which are isolated from each other bythe isolation regions, on the semiconductor substrate; forming atantalum oxide film and a silicon dioxide film in the first and secondregions on the semiconductor substrate, respectively; forming arelatively thick silicon dioxide film and a relatively thin silicondioxide film in the second and third regions on the semiconductorsubstrate, respectively, by heat-treating the semiconductor substratewithin an ambient containing oxygen as a main component; forming first,second and third gate electrodes on the tantalum oxide, thick silicondioxide and thin silicon dioxide films, respectively; and forming first,second and third gate insulating films by etching the tantalum oxide,thick silicon dioxide and thin silicon dioxide films using the first,second and third gate electrodes as respective masks.

According to the second method for fabricating a semiconductor device, asemiconductor substrate, on which a tantalum oxide film and a silicondioxide films have been formed in first and second regions,respectively, is heat-treated within an ambient containing oxygen as amain component. Thus, the tantalum oxide film is left in the firstregion, the silicon dioxide film grows to form a thick silicon dioxidefilm in the second region and a thin silicon dioxide film is newlyformed in the third region. Thereafter, first, second and third gateinsulating films are formed by etching the tantalum oxide, thick silicondioxide and thin silicon dioxide films using first, second and thirdgate electrodes as respective masks. In this manner, the first, secondand third gate insulating films can be formed out of the tantalum oxide,thick silicon dioxide and thin silicon dioxide films, respectively, onthe same semiconductor substrate without increasing the number ofheat-treating or masking process steps.

To achieve the second object, a third method for fabricating asemiconductor device according to the present invention includes thesteps of: forming an isolation region on a surface portion of asemiconductor substrate of silicon, thereby defining first and secondregions, which are isolated from each other by the isolation region, onthe semiconductor substrate; forming a capacitive lower electrode ineach of the first and second regions on the semiconductor substrate;forming a silicon nitride film on the capacitive lower electrode in eachof the first and second regions; forming a tantalum oxide film on thesilicon nitride film in the first region, thereby forming a firstcapacitive insulating film as the stack of the silicon nitride andtantalum oxide films; heat-treating the semiconductor substrate withinan ambient containing oxygen as a main component to form a silicondioxide film on a surface portion of the silicon nitride film in thesecond region, thereby forming a second capacitive insulating film asthe stack of the silicon nitride and silicon dioxide films; and forminga capacitive upper electrode on the first and second capacitiveinsulating films.

According to the third method for fabricating a semiconductor device, asemiconductor substrate, on which a first capacitive insulating film hasbeen formed as a stack of silicon nitride and tantalum oxide films on acapacitive lower electrode in a first region and a silicon nitride filmhas been formed on a capacitive lower electrode in a second region, isheat-treated within an ambient containing oxygen as a main component.Accordingly, the first capacitive insulating film as the stack of thesilicon nitride and tantalum oxide films is left in the first region,while a second capacitive insulating film is formed in the second regionas a stack of the silicon nitride and silicon dioxide films. Thus, thefirst and second capacitive insulating films can be formed as the stackof the silicon nitride and tantalum oxide films and the stack of thesilicon nitride and silicon dioxide films, respectively, on the samesemiconductor substrate without increasing the number of heat-treatingor masking process steps.

To achieve the first object, a fourth method for fabricating asemiconductor device according to the present invention includes thesteps of: forming an isolation region on a surface portion of asemiconductor substrate of silicon, thereby defining first and secondregions, which are isolated from each other by the isolation region, onthe semiconductor substrate; forming a tantalum oxide film and a silicondioxide film in the first and second regions on the semiconductorsubstrate, respectively; forming a tantalum nitride film in a surfaceportion of the tantalum oxide film and changing the silicon dioxide filminto a silicon oxynitride film by heat-treating the semiconductorsubstrate within an ambient containing nitrogen as a main component;forming a first gate electrode on the stack of the tantalum oxide andnitride films and a second gate electrode on the silicon oxynitridefilm, respectively; and forming first and second gate insulating filmsby etching the stack of the tantalum oxide and nitride films and thesilicon oxynitride film using the first and second gate electrodes asrespective masks.

According to the fourth method for fabricating a semiconductor device, asemiconductor substrate, on which tantalum oxide and silicon dioxidefilms have been formed in first and second regions, respectively, isheat-treated within an ambient containing nitrogen as a main component.Thus, a tantalum nitride film is formed on the tantalum oxide film inthe first region, while the silicon dioxide film changes into a siliconoxynitride film in the second region. Thereafter, first and second gateinsulating films are formed by etching the stack of the tantalum oxideand nitride films and the silicon oxynitride film using the first andsecond gate electrodes as respective masks. In this manner, the firstand second gate insulating films can be formed out of the stack of thetantalum oxide and nitride films and the silicon oxynitride film,respectively, on the same semiconductor substrate without increasing thenumber of heat-treating or masking process steps.

To achieve the first object, a fifth method for fabricating asemiconductor device according to the present invention includes thesteps of: forming an isolation region on a surface portion of asemiconductor substrate of silicon, thereby defining first and secondregions, which are isolated from each other by the isolation region, onthe semiconductor substrate; forming a tantalum oxide film in the firstregion on the semiconductor substrate; forming a tantalum nitride filmin a surface portion of the tantalum oxide film and a silicon oxynitridefilm in the second region on the semiconductor substrate, respectively,by heat-treating the semiconductor substrate within an ambientcontaining oxygen and nitrogen as main components; forming a first gateelectrode on the stack of the tantalum oxide and nitride films and asecond gate electrode on the silicon oxynitride film, respectively; andforming first and second gate insulating films by etching the stack ofthe tantalum oxide and nitride films and the silicon oxynitride filmusing the first and second gate electrodes as respective masks.

According to the fifth method for fabricating a semiconductor device, asemiconductor substrate, on which a tantalum oxide film has been formedin a first region, is heat-treated within an ambient containing oxygenand nitrogen as main components. Thus, a tantalum nitride film is formedon the tantalum oxide film in the first region, while a siliconoxynitride film is formed in the second region. Thereafter, first andsecond gate insulating films are formed by etching the stack of thetantalum oxide and nitride films and the silicon oxynitride film usingthe first and second gate electrodes as respective masks. In thismanner, the first and second gate insulating films can be formed out ofthe stack of the tantalum oxide and nitride films and the siliconoxynitride film, respectively, on the same semiconductor substratewithout increasing the number of heat-treating or masking process steps.

To achieve the first object, a sixth method for fabricating asemiconductor device according to the present invention includes thesteps of: forming an isolation region on a surface portion of asemiconductor substrate of silicon, thereby defining first and secondregions, which are isolated from each other by the isolation region, onthe semiconductor substrate; forming a silicon nitride film in each ofthe first and second regions on the semiconductor substrate; forming atantalum oxide film on the silicon nitride film in the first region;changing the silicon nitride film in the second region into a stack ofthe silicon nitride film and a silicon dioxide film or into a siliconoxynitride film by heat-treating the semiconductor substrate within anambient containing oxygen as a main component; forming a first gateelectrode on the stack of the silicon nitride and tantalum oxide filmsand a second gate electrode on the stack of the silicon nitride andsilicon dioxide films or the silicon oxynitride film, respectively; andforming first and second gate insulating films by etching the stack ofthe silicon nitride and tantalum oxide films and the stack of thesilicon nitride and silicon dioxide films or the silicon oxynitride filmusing the first and second gate electrodes as respective masks.

According to the sixth method for fabricating a semiconductor device, asemiconductor substrate, on which a stack of silicon nitride andtantalum oxide films has been formed in a first region and a siliconnitride film has been formed in a second region, is heat-treated withinan ambient containing oxygen as a main component. Thus, the stack of thesilicon nitride and tantalum oxide films is left in the first region,while the silicon nitride film changes into a stack of the siliconnitride and silicon dioxide films or a silicon oxynitride film in thesecond region. Thereafter, first and second gate insulating films areformed by etching the stack of the silicon nitride and tantalum oxidefilms and the stack of the silicon nitride and silicon dioxide films orthe silicon oxynitride film using the first and second gate electrodesas respective masks. In this manner, the first and second gateinsulating films can be formed out of the stack of the silicon nitrideand tantalum oxide films and the stack of the silicon nitride andsilicon dioxide films or the silicon oxynitride film, respectively, onthe same semiconductor substrate without increasing the number ofheat-treating or masking process steps.

To achieve the first object, a seventh method for fabricating asemiconductor device according to the present invention includes thesteps of: forming an isolation region on a surface portion of asemiconductor substrate of silicon, thereby defining first and secondregions, which are isolated from each other by the isolation region, onthe semiconductor substrate; forming a silicon dioxide film in thesecond region on the semiconductor substrate; forming a silicon nitridefilm in the first region on the semiconductor substrate and changing thesilicon dioxide film into a silicon oxynitride film by heat-treating thesemiconductor substrate within an ambient containing nitrogen as a maincomponent; forming a tantalum oxide film on the silicon nitride film;forming a first gate electrode on the stack of the silicon nitride andtantalum oxide films and a second gate electrode on the siliconoxynitride film, respectively; and forming first and second gateinsulating films by etching the stack of the silicon nitride andtantalum oxide films and the silicon oxynitride film using the first andsecond gate electrodes as respective masks.

According to the seventh method for fabricating a semiconductor device,a semiconductor substrate, on which a silicon dioxide film has beenformed in a second region, is heat-treated within an ambient containingnitrogen as a main component. Thus, a silicon nitride film is formed inthe first region, while the silicon dioxide film changes into a siliconoxynitride film in the second region. Thereafter, a tantalum oxide filmis formed on the silicon nitride film, thereby forming a stack of thesilicon nitride and tantalum oxide films in the first region. Then,first and second gate insulating films are formed by etching the stackof the silicon nitride and tantalum oxide films and the siliconoxynitride film using the first and second gate electrodes as respectivemasks. In this manner, the first and second gate insulating films can beformed out of the stack of the silicon nitride and tantalum oxide filmsand the silicon oxynitride film, respectively, on the same semiconductorsubstrate without increasing the number of heat-treating or maskingprocess steps.

Thus, according to the first through seventh methods for fabricating asemiconductor device, a semiconductor device, which includes MOStransistors with different types of gate insulating films or capacitorswith different kinds of capacitive insulating films and does not haveits performance deteriorated due to the increase in number ofheat-treating process steps, can be fabricated just as intended withoutincreasing the number of heat-treating or masking process steps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a), 1(b) and 1(c) are cross-sectional views illustratingrespective process steps for fabricating a semiconductor deviceaccording to a first embodiment of the present invention.

FIGS. 2, 3 and 4 are cross-sectional views illustrating respectiveprocess steps for fabricating a semiconductor device according to asecond embodiment of the present invention.

FIGS. 5(a), 5(b) and 5(c) are cross-sectional views illustratingrespective process steps for fabricating a semiconductor deviceaccording to a third embodiment of the present invention.

FIGS. 6(a) and 6(b) are cross-sectional views illustrating respectiveprocess steps for fabricating the semiconductor device according to thethird embodiment of the present invention.

FIGS. 7(a), 7(b) and 7(c) are cross-sectional views illustratingrespective process steps for fabricating a semiconductor deviceaccording to a fourth embodiment of the present invention.

FIGS. 8(a) and 8(b) are cross-sectional views illustrating respectiveprocess steps for fabricating a semiconductor device according to afifth embodiment of the present invention.

FIGS. 9(a), 9(b) and 9(c) are cross-sectional views illustratingrespective process steps for fabricating a semiconductor deviceaccording to a sixth embodiment of the present invention.

FIGS. 10(a) and 10(b) are cross-sectional views illustrating respectiveprocess steps for fabricating a semiconductor device according to aseventh embodiment of the present invention.

FIGS. 11(a), 11(b), 11(c) and 11(d) are cross-sectional viewsillustrating respective process steps for fabricating a semiconductordevice according to a first prior art example.

FIG. 12 is a cross-sectional view illustrating a process step forfabricating a semiconductor device according to a second prior artexample.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

Hereinafter, a method for fabricating a semiconductor device accordingto a first embodiment will be described with reference to FIGS. 1(a),1(b) and 1(c). In the method according to the first embodiment, twotypes of gate insulating films can be formed out of tantalum oxide andsilicon dioxide films without increasing the number of masking orheat-treating process steps when MOS transistors are formed atpredetermined locations on a semiconductor substrate. FIGS. 1(a) through1(c) illustrate an embodiment where a gate insulating film is formed outof a tantalum oxide film in a first region on the left-hand side andanother gate insulating film is formed out of a silicon dioxide film ina second region on the right-hand side.

First, as shown in FIG. 1(a), isolation regions 101 are defined onrespective surface portions of a semiconductor substrate 100 of silicon,and then a tantalum oxide film 102 is deposited in the first region to athickness of 10 nm, for example, by a CVD process, for instance.

Next, the semiconductor substrate 100 is heat-treated at 900° C. for 30minutes, for example, within an ambient containing oxygen as a maincomponent. As a result, the tantalum oxide film 102 in the first regionchanges from an amorphous state, in which carbon atoms causing thedeterioration of the film properties exist in the film, into acrystallized state, in which those carbon atoms have been removedtherefrom. In the second region on the other hand, the surface region ofthe semiconductor substrate 100 of silicon is oxidized to form a silicondioxide film 103 as shown in FIG. 1(b) .

Subsequently, a conductor film is deposited over the tantalum oxide andsilicon dioxide films 102 and 103 and then patterned into gate electrodeshapes. In this manner, first and second gate electrodes 104 and 105 areformed on the tantalum oxide and silicon dioxide films 102 and 103,respectively, as shown in FIG. 1(c). Thereafter, the tantalum oxide andsilicon dioxide films 102 and 103 are etched using the first and secondgate electrodes 104 and 105 as respective masks, thereby forming firstand second gate insulating films 106 and 107.

As described above, according to the first embodiment, the semiconductorsubstrate 100, on which the tantalum oxide film 102 has been depositedin the first region, is heat-treated within the ambient containingoxygen as a main component. In this manner, the tantalum oxide film 102is crystallized in the first region to remove carbon therefrom, whilethe silicon dioxide film 103 is newly formed in the second region. Thus,two types of MOS transistors with different kinds of gate insulatingfilms can be formed on the same semiconductor substrate 100 withoutincreasing the number of heat-treating or masking process steps.

Similar effects are attainable if the semiconductor substrate 100 isheat-treated at 900° C. for about 60 seconds, for example, using a rapidthermal annealing furnace, instead of being heat-treated at 900° C. for30 minutes.

Embodiment 2

Hereinafter, a method for fabricating a semiconductor device accordingto a second embodiment will be described with reference to FIGS. 2through 4. In the method according to the second embodiment, three typesof gate insulating films can be formed out of tantalum oxide, thicksilicon dioxide and thin silicon dioxide films without increasing thenumber of masking or heat-treating process steps when MOS transistorsare formed at predetermined regions on a semiconductor substrate. FIGS.2 through 4 illustrate an embodiment where a gate insulating film isformed out of the tantalum oxide film in a first region on the left-handside, another gate insulating film is formed out of the thick silicondioxide film in a second region at the center and still another gateinsulating film is formed out of the thin silicon dioxide film in athird region on the right-hand side.

First, as shown in FIG. 2, isolation regions 201 are defined onrespective surface portions of a semiconductor substrate 200 of silicon.Then, a tantalum oxide film 202 is deposited in the first region to athickness of 10 nm, for example, by a CVD process, for instance. And asilicon dioxide film 203 is formed in the second region to a thicknessof 5 nm, for example, by a thermal oxidation or CVD process.

Next, the semiconductor substrate 200 is heat-treated at 900° C. for 30minutes, for example, within an ambient containing oxygen as a maincomponent. As a result, the tantalum oxide film 202 in the first regionchanges from an amorphous state, in which carbon atoms causing thedeterioration of the film properties exist in the film, into acrystallized state, in which those carbon atoms have been removedtherefrom. In the second region on the other hand, the silicon dioxidefilm 203 grows to a thickness of about 15 nm to form a thick silicondioxide film 204. Furthermore, in the third region, the surface regionof the semiconductor substrate 200 of silicon is oxidized to form a thinsilicon dioxide film 205 with a thickness of about 10 nm as shown inFIG. 3.

Subsequently, a conductor film is deposited over the entire surfaceincluding the tantalum, thick silicon dioxide and thin silicon dioxidefilms 202, 204 and 205 and then patterned into gate electrode shapes. Inthis manner, first, second and third gate electrodes 206, 207 and 208are formed on the tantalum oxide, thick silicon dioxide and thin silicondioxide films 202, 204 and 205, respectively, as shown in FIG. 4.Thereafter, the tantalum oxide, thick silicon dioxide and thin silicondioxide films 202, 204 and 205 are etched using the first, second andthird gate electrodes 206, 207 and 208 as masks, thereby forming first,second and third gate insulating films 209, 210 and 211, respectively.

As described above, according to the second embodiment, thesemiconductor substrate 200, on which the tantalum oxide and silicondioxide films 202 and 203 have been formed in the first and secondregions, respectively, is heat-treated within the ambient containingoxygen as a main component. In this manner, the tantalum oxide film 202is crystallized in the first region to remove carbon therefrom, thesilicon dioxide film 203 is grown into the thick silicon dioxide film204 in the second region and the thin silicon dioxide film 205 is newlyformed in the third region. Accordingly, three types of MOS transistorswith gate insulating films of different kinds or thicknesses can beformed on the same semiconductor substrate 200 without increasing thenumber of heat-treating or masking process steps.

Similar effects are attainable if the semiconductor substrate 200 isheat-treated at 900° C. for about 60 seconds, for example, using a rapidthermal annealing furnace, instead of being heat-treated at 900° C. for30 minutes.

Alternatively, the thick and thin silicon dioxide films 204 and 205 maybe formed in the following manner. First, a mask is defined over thethird region of the semiconductor substrate 200. Then, the semiconductorsubstrate 200 is heat-treated within an ambient containing oxygen as amain component, thereby forming the silicon dioxide film 203 in thesecond region. Thereafter, the mask is removed and then thesemiconductor substrate 200 is heat-treated again within the ambientcontaining oxygen as a main component such that the thick and thinsilicon dioxide films 204 and 205 are formed in the second and thirdregions, respectively.

Embodiment 3

Hereinafter, a method for fabricating a semiconductor device accordingto a third embodiment will be described with reference to FIGS. 5(a),5(b) and 5(c) and FIGS. 6(a) and 6(b). In the method according to thethird embodiment, two types of capacitive insulating films can be formedout of a stack of tantalum oxide and silicon nitride films and a stackof silicon dioxide and nitride films, respectively, without increasingthe number of masking or heat-treating process steps when a storagecapacitor portion is formed for a semiconductor storage device at apredetermined region on a semiconductor substrate. FIGS. 5(a) through5(c) and FIGS. 6(a) and 6(b) illustrate an embodiment where a capacitiveinsulating film is formed out of a stack of silicon nitride and tantalumoxide films in a first region on the left-hand side and anothercapacitive insulating film is formed out of a stack of silicon nitrideand silicon dioxide films in a second region on the right-hand side.

First, as shown in FIG. 5(a), isolation regions 301 are defined onrespective surface portions of a semiconductor substrate 300 of silicon,an interlevel dielectric film 302 is deposited over the entire surfacethereof and then contact holes 303 are formed in the interleveldielectric film 302. Next, a conductive polysilicon film is deposited toa thickness of 500 nm, for example, over the entire surface of theinterlevel dielectric film 302 and then patterned, thereby formingcapacitive lower electrodes 304.

Then, respective surface regions of the capacitive lower electrodes 304are nitrided or a silicon nitride film is deposited thereon by a CVDprocess. As a result, a silicon nitride film 305 is formed to athickness of 6 nm, for example, on the surface of each capacitive lowerelectrode 304 as shown in FIG. 5(b) .

Subsequently, as shown in FIG. 5(c), a tantalum oxide film 306 isselectively deposited to a thickness of 10 nm, for example, on thesilicon nitride film 305 that has been formed in the first region. Inthis manner, a capacitive insulating film is formed as stack of thesilicon nitride and tantalum oxide films 305 and 306.

Next, the semiconductor substrate 300 is heat-treated at 900° C. for 30minutes, for example, within an ambient containing oxygen as a maincomponent. As a result, the tantalum oxide film 306 in the first regionchanges from an amorphous state, in which carbon atoms causing thedeterioration of the film properties exist in the film, into acrystallized state, in which those carbon atoms have been removedtherefrom. In the second region on the other hand, the surface region ofthe silicon nitride film 305 is oxidized to form a capacitive insulatingfilm as a stack of the silicon nitride and silicon dioxide films 305 and307 as shown in FIG. 6(a).

Next, a conductive polysilicon film is deposited over the entire surfaceof the semiconductor substrate 300 and then patterned to form acapacitive upper electrode 308 as shown in FIG. 6(b).

As described above, according to the third embodiment, the semiconductorsubstrate 300, on which the tantalum oxide film 306 has been formed inthe first region and the silicon nitride film 305 has been formed in thesecond region, is heat-treated within the ambient containing oxygen as amain component. In this manner, the tantalum oxide film 306 iscrystallized in the first region to remove carbon therefrom, while thestack of the silicon nitride and silicon dioxide films 305 and 307 isformed in the second region. Thus, two types of semiconductor storagedevices with different kinds of capacitive insulating films can beformed on the same semiconductor substrate 300 without increasing thenumber of heat-treating or masking process steps.

Similar effects are attainable if the semiconductor substrate 300 isheat-treated at 900° C. for about 60 seconds, for example, using a rapidthermal annealing furnace, instead of being heat-treated at 900° C. for30 minutes.

Embodiment 4

Hereinafter, a method for fabricating a semiconductor device accordingto a fourth embodiment will be described with reference to FIGS. 7(a),7(b) and 7(c). In the method according to the fourth embodiment, twotypes of gate insulating films can be formed out of a stack of tantalumnitride and oxide films and a silicon oxynitride film without increasingthe number of masking or heat-treating process steps when MOStransistors are formed at predetermined locations on a semiconductorsubstrate. FIGS. 7(a) through 7(c) illustrate an embodiment where a gateinsulating film with good barrier properties is formed out of a stack ofthe tantalum nitride and oxide films in a first region on the left-handside and another gate insulating film is formed out of the siliconoxynitride film in a second region on the right-hand side.

First, as shown in FIG. 7(a), isolation regions 401 are defined onrespective surface portions of a semiconductor substrate 400 of silicon.Then, a tantalum oxide film 402 is deposited in the first region to athickness of 10 nm, for example, by a CVD process, for instance, and asilicon dioxide film 403 is deposited in the second region to athickness of 3 nm, for example.

Next, the semiconductor substrate 400 is heat-treated at 900° C. for 10minutes, for example, within an ambient containing nitrogen as a maincomponent, e.g., ammonium ambient. As a result, a tantalum nitride film404 is formed on the surface of the tantalum oxide film 402 in the firstregion, while the silicon dioxide film 403 changes into a siliconoxynitride film 405 in the second region. In this case, the tantalumoxide film 402 in the first region changes from an amorphous state, inwhich carbon atoms exist in the film, into a crystallized state, inwhich those carbon atoms have been removed therefrom.

Subsequently, a conductor film is deposited over the stack of thetantalum oxide and nitride films 402 and 404 and over the siliconoxynitride film 405 and then patterned into gate electrode shapes. Inthis manner, first and second gate electrodes 406 and 407 are formed onthe stack of the tantalum oxide and nitride films 402 and 404 and on thesilicon oxynitride film 405, respectively, as shown in FIG. 7(c).Thereafter, the stack of the tantalum oxide and nitride films 402 and404 and the silicon oxynitride film 405 are etched using the first andsecond gate electrodes 406 and 407 as respective masks, thereby formingfirst and second gate insulating films 408 and 409.

As described above, according to the fourth embodiment, thesemiconductor substrate 400, on which the tantalum oxide film 402 andsilicon dioxide film 403 have been formed in the first and secondregions, respectively, is heat-treated within the ambient containingnitrogen as a main component, e.g., ammonium ambient. In this manner,the tantalum oxide film 402 is crystallized in the first region toremove carbon therefrom and the tantalum nitride film 404 is formed onthe tantalum oxide film 402. On the other hand, the silicon dioxide film403 is changed into the silicon oxynitride film 405 in the secondregion. Thus, two types of MOS transistors with different kinds of gateinsulating films can be formed on the same semiconductor substrate 400without increasing the number of heat-treating or masking process steps.

Similar effects are attainable if the semiconductor substrate 400 isheat-treated within an ambient containing nitrogen gas instead ofammonium.

Also, the tantalum oxide and silicon dioxide films 402 and 403 shown inFIG. 7(a) may be formed by the method according to the first embodiment.

Embodiment 5

Hereinafter, a method for fabricating a semiconductor device accordingto a fifth embodiment will be described with reference to FIGS. 8(a) and8(b). In the method according to the fifth embodiment, two types of gateinsulating films can be formed out of a stack of tantalum oxide andnitride films and a silicon oxynitride film without increasing thenumber of masking or heat-treating process steps when MOS transistorsare formed at predetermined locations on a semiconductor substrate.FIGS. 8(a) and 8(b) illustrate an embodiment where a gate insulatingfilm with good barrier properties is formed out of the stack of thetantalum oxide and nitride films in a first region on the left-hand sideand another gate insulating film is formed out of the silicon oxynitridefilm in a second region on the right-hand side.

First, as shown in FIG. 8(a), isolation regions 501 are defined onrespective surface portions of a semiconductor substrate 500 of silicon.Then, a tantalum oxide film 502 is deposited in the first region to athickness of 10 nm, for example, by a CVD process, for instance.

Next, the semiconductor substrate 500 is heat-treated at 900° C. for 10minutes, for example, within an ambient containing oxygen and nitrogenas main components, e.g., an ambient containing nitrogen monoxide gas.As a result, a tantalum nitride film 503 with barrier properties isformed in the first region on the surface of the tantalum oxide film 502as shown in FIG. 8(b). In addition, the tantalum oxide film 502 changesfrom an amorphous state, in which carbon atoms exist in the film, into acrystallized state, in which those carbon atoms have been removedtherefrom. In the second region on the other hand, the surface region ofthe semiconductor substrate 500 is oxidized and nitrided to form asilicon oxynitride film 504 to a thickness of 15 nm.

Subsequently, although not shown, the first and second gate electrodesand first and second gate insulating films are formed as is done in thefourth embodiment.

As described above, according to the fifth embodiment, the semiconductorsubstrate 500, on which the tantalum oxide film 502 has been formed inthe first region, is heat-treated within the ambient containing oxygenand nitrogen as main components, e.g., nitrogen monoxide ambient. Inthis manner, the tantalum oxide film 502 is crystallized in the firstregion to remove carbon therefrom and the tantalum nitride film 503 isformed on the tantalum oxide film 502. In the second region on the otherhand, the silicon oxynitride film 504 is formed. Thus, two types of MOStransistors with different kinds of gate insulating films can be formedon the same semiconductor substrate 500 without increasing the number ofheat-treating or masking process steps.

Similar effects are attainable if the semiconductor substrate 500 isheat-treated at 900° C. for about 30 seconds, for example, using a rapidthermal annealing furnace, instead of being heat-treated at 900° C. for10 minutes. Similar effects are also attainable even if thesemiconductor substrate 500 is heat-treated within an ambient containingnitrogen dioxide gas, not the nitrogen monoxide ambient.

Embodiment 6

Hereinafter, a method for fabricating a semiconductor device accordingto a sixth embodiment will be described with reference to FIGS. 9(a),9(b) and 9(c). In the method according to the sixth embodiment, twotypes of gate insulating films can be formed out of a stack of tantalumoxide and silicon nitride films and a silicon oxynitride film withoutincreasing the number of masking or heat-treating process steps when MOStransistors are formed at predetermined locations on a semiconductorsubstrate. FIGS. 9(a) through 9(c) illustrate an embodiment where a gateinsulating film with good barrier properties is formed out of the stackof the tantalum oxide and silicon nitride films in a first region on theleft-hand side and another gate insulating film is formed out of thesilicon oxynitride film in a second region on the right-hand side.

First, as shown in FIG. 9(a), isolation regions 601 are defined onrespective surface portions of a semiconductor substrate 600 of silicon.Then, the semiconductor substrate 600 is heat-treated at 900° C. for 10minutes, for example, within an ambient containing nitrogen as a maincomponent, e.g., ammonium ambient, thereby forming a silicon nitridefilm 602 in the first region.

Next, as shown in FIG. 9(b), a tantalum oxide film 603 is selectivelydeposited to a thickness of 10 nm, for example, on the silicon nitridefilm 602 in the first region by a CVD process, for instance. Thereafter,the semiconductor substrate 600 is heat-treated at 900° C. for 30minutes, for example, within an ambient containing oxygen as a maincomponent. As a result, the tantalum oxide film 603 in the first regionchanges from an amorphous state, in which carbon atoms exist in thefilm, into a crystallized state, in which those carbon atoms have beenremoved therefrom. In the second region on the other hand, the siliconnitride film 602 changes into a silicon oxynitride film 604 or a stackof silicon nitride and dioxide films.

Subsequently, a conductor film is deposited over the stack of thesilicon nitride and tantalum oxide films 602 and 603 and over thesilicon oxynitride film 604 and then patterned into gate electrodeshapes. In this manner, first and second gate electrodes 605 and 606 areformed on the stack of the silicon nitride and tantalum oxide films 602and 603 and on the silicon oxynitride film 604, respectively, as shownin FIG. 9(c). Thereafter, the stack of the silicon nitride and tantalumoxide films 602 and 603 and the silicon oxynitride film 604 are etchedusing the first and second gate electrodes 605 and 606 as respectivemasks, thereby forming first and second gate insulating films 607 and608.

As described above, according to the sixth embodiment, the semiconductorsubstrate 600, on which the stack of the silicon nitride and tantalumoxide films 602 and 603 has been formed in the first region, isheat-treated within the ambient containing oxygen as a main component.In this manner, the tantalum oxide film 603 is crystallized in the firstregion to remove carbon therefrom. In the second region on the otherhand, the silicon nitride film 602 is changed into the siliconoxynitride film 604. Thus, two types of MOS transistors with differentkinds of gate insulating films can be formed on the same semiconductorsubstrate 600 without increasing the number of heat-treating or maskingprocess steps.

Similar effects are attainable if the semiconductor substrate 600 isheat-treated at 900° C. for about 30 seconds, for example, using a rapidthermal annealing furnace, instead of being heat-treated at 900° C. for10 minutes. Similar effects are also attainable even if thesemiconductor substrate 600 is heat-treated within an ambient containingnitrogen gas or nitrogen oxide gas, not the ambient containing ammonium.

Embodiment 7

Hereinafter, a method for fabricating a semiconductor device accordingto a seventh embodiment will be described with reference to FIGS. 10(a)and 10(b). In the method according to the seventh embodiment, two typesof gate insulating films can be formed out of a stack of tantalum oxideand silicon nitride films and a silicon oxynitride film withoutincreasing the number of masking or heat-treating process steps when MOStransistors are formed at predetermined locations on a semiconductorsubstrate. FIGS. 10(a) and 10(b) illustrate an embodiment where a gateinsulating film with good barrier properties is formed out of the stackof the tantalum oxide and silicon nitride films in a first region on theleft-hand side and another gate insulating film is formed out of thesilicon oxynitride film in a second region on the right-hand side.

First, as shown in FIG. 10(a), isolation regions 701 are defined onrespective surface portions of a semiconductor substrate 700 of silicon.Then, a silicon dioxide film 702 is deposited in the second region to athickness of 3 nm, for example.

Next, the semiconductor substrate 700 is heat-treated at 900° C. for 10minutes, for example, within an ambient containing nitrogen as a maincomponent, e.g., ammonium ambient. As a result, the surface region ofthe semiconductor substrate 700 is nitrided to form a silicon nitridefilm 703 in the first region. In the second region on the other hand,the silicon dioxide film 702 is nitrided and changed into a siliconoxynitride film 704. Thereafter, a tantalum oxide film 705 is depositedon the silicon nitride film 703 in the first region by a CVD process,for example.

Subsequently, although not shown, the first and second gate electrodesand first and second gate insulating films are formed as in the sixthembodiment.

As described above, according to the seventh embodiment, after thesilicon dioxide film 702 has been formed in the second region, heattreatment is conducted within the ambient containing nitrogen as a maincomponent, e.g., ammonium ambient. As a result, the surface region ofthe semiconductor substrate 700 is nitrided in the first region to formthe silicon nitride film 703. In the second region on the other hand,the silicon dioxide film 702 changes into the silicon oxynitride film704. Thereafter, the tantalum oxide film 705 is deposited on the siliconnitride film 703 in the first region. Thus, two types of MOS transistorswith different kinds of gate insulating films can be formed on the samesemiconductor substrate 700 without increasing the number ofheat-treating or masking process steps.

Similar effects are attainable if the semiconductor substrate 700 isheat-treated at 900° C. for about 30 seconds , for example, using arapid thermal annealing furnace, instead of being heat-treated at 900°C. for 10 minutes. Similar effects are also attainable even if thesemiconductor substrate 700 is heat-treated within an ambient containingnitrogen gas or nitrogen oxide gas, not the ammonium ambient.

What is claimed is:
 1. A method for fabricating a semiconductor device,comprising the steps of: forming an isolation region on a surfaceportion of a semiconductor substrate of silicon, thereby defining firstand second regions, which are isolated from each other by the isolationregion, on the semiconductor substrate; forming a tantalum oxide filmonly in the first region on the semiconductor substrate; forming asilicon dioxide film in the second region on the semiconductor substrateby heat-treating the semiconductor substrate within an ambientcontaining oxygen as a main component; forming first and second gateelectrodes on the tantalum oxide and silicon dioxide films,respectively; and forming first and second gate insulating films byetching the tantalum oxide and silicon dioxide films using the first andsecond gate electrodes as respective masks.
 2. A method for fabricatinga semiconductor device, comprising the steps of: forming a plurality ofisolation regions on respective surface portions of a semiconductorsubstrate of silicon, thereby defining first, second and third regions,which are isolated from each other by the isolation regions, on thesemiconductor substrate; forming a tantalum oxide film and a silicondioxide film only in the first and second regions on the semiconductorsubstrate, respectively; forming a relatively thick silicon dioxide filmand a relatively thin silicon dioxide film in the second and thirdregions on the semiconductor substrate, respectively, by heat-treatingthe semiconductor substrate within an ambient containing oxygen as amain component; forming first, second and third gate electrodes on thetantalum oxide, thick silicon dioxide and thin silicon dioxide films,respectively; and forming first, second and third gate insulating filmsby etching the tantalum oxide, thick silicon dioxide and thin silicondioxide films using the first, second and third gate electrodes asrespective masks.
 3. A method for fabricating a semiconductor device,comprising the steps of: forming an isolation region on a surfaceportion of a semiconductor substrate of silicon, thereby defining firstand second regions, which are isolated from each other by the isolationregion, on the semiconductor substrate; forming a silicon nitride filmin each of the first and second regions on the semiconductor substrate;forming a tantalum oxide film only on the silicon nitride film in thefirst region; changing the silicon nitride film in the second regioninto a stack of the silicon nitride film and a silicon dioxide film orinto a silicon oxynitride film by heat-treating the semiconductorsubstrate within an ambient containing oxygen as a main component;forming a first gate electrode on the stack of the silicon nitride andtantalum oxide films and a second gate electrode on the stack of thesilicon nitride and silicon dioxide films or the silicon oxynitridefilm, respectively; and forming first and second gate insulating filmsby etching the stack of the silicon nitride and tantalum oxide films andthe stack of the silicon nitride and silicon dioxide films or thesilicon oxynitride film using the first and second gate electrodes asrespective masks.